Thermal energy can be usefully converted into mechanical and then electrical form. Methods of converting the thermal energy of low and high temperature heat sources into electric power present an important area of energy generation. Thermal energy from a heat source can be transformed into mechanical and then electrical form using a working stream that is expanded and regenerated in a closed system operating on a thermodynamic cycle. The working fluid can include components of different boiling temperatures, and the composition of the working fluid can be modified at different places within the system to improve the efficiency of energy conversion operation. In any process of converting thermal energy to a usable form, the major loss of available energy in the heat source occurs in the process of boiling or evaporating the working fluid. This loss of available energy known as exergy or energy is due to the mismatch of the enthalpy-temperature characteristics of the heat source and the working fluid in the boiler. Simply put, for any given enthalpy, the temperature of the heat source is always greater than the temperature of the working fluid. The use of a mixture as a working fluid as in the Kalina cycle reduces these losses to a significant extent. Also, the use of low temperature heat sources for heating the working fluid to reduce such losses has been established. However, it would be highly desirable to further reduce these losses in any cycle.
U.S. Pat. No. 4,573,321 discloses power generating cycle which permits the extraction of energy from low temperature heat sources. The vaporous working fluid is withdrawn from the single stage distillation section and expanded in a turbine. The expanded working fluid is condensed in a direct contact condenser or absorber. The separated weak solution from the phase distillation column exchanges heat with the condensed working fluid and is reheated in a regenerator and trim heater. However, it uses heat from an external heat source at the regenerator and the trim heater. Also, the first law efficiency as reported in this patent is as low as 8.5% and second law efficiency is as low as 45%.
U.S. Pat. No. 5,029,444 discloses a thermodynamic cycle utilizing low temperature variable heat source at 110° C. to 77° C. The spent stream formed after the expansion of gaseous working stream is mixed with the lean stream to form pre-condensed stream. The pre-condensed stream is further condensed to produce a liquid working stream. The liquid working stream is then partially evaporated, utilizing heat of the spent stream and the lean stream. After phase separations, liquid stream is mixed with the vapor stream from reboiler to produce the enriched stream. This stream is in state of vapor-liquid mixture that is then heated with heat source to form gaseous working stream. The gaseous working stream is then expanded in expander to produce the usable form of energy. However, the patent reports second law efficiency of only 49.66%. Here, the system is operated at a very low pressure and less pressure ratio for isentropic operation in turbine, which results in low power output and inefficient heat integration.
Accordingly, a need exists for a thermodynamic cycle which can increase the efficiency, improve the heat utilization in the cycle by best possible heat integration that would result in much better heat recovery.